Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: coquina.rocks on 27/08/2009 14:10:50
-
Hi y'all,
(The classic greeting given by a GRIT - Girl Raised In The South)
I'm new here, a baby-boomer, and am particularly interested in Geology. I live in Southeastern Virginia, USA, and on top of the Chesapeake Bay Impact Crater, the top of which is approximately 400' below the surface. At the end of this post I have attached a link to a USGS website about the crater. More technical papers are available, and more research has been done since it was created, but it is the most comprehensive and easiest to understand of all the sites, and has wonderful graphics which will enlarge with a "click".
I have been studying the crater since 1996, and have met (and been encouraged by) several of the geologists who discovered the crater. They have even allowed me on the drilling sites and allowed me to help pull a core (one of the highlights of my life, to be the first of two humans to ever lay eyes on sediments laid down up to 65 million years ago).
People have always been perplexed as to some of the goings-on around here. For example, why is it, that no matter how well houses are constructed, or the quality of the foundation, plaster walls crack? Why does sea level seem to be rising here faster than it does in other places? When a well is drilled, why is it that the water withdrawn may be 1.5 times saltier than the ocean? Why are exposed strata in various localities tilted even though the area is aseismic?
Even more perplexing to early hydrologists and geologists, when they examined well tailings below 400', why were telltales age markers form 35 to 65 MYA intermingled? Cederstrom, a USGS hydrologist, named the intermingled strata the "Mattaponi Formation" and suspected it was caused by a catastrophe, but his theory was not accepted.
In 1983, the Glomar Challenger retrieved a core off the New Jersey coast that contained both tektites and shocked quartz - indicators of an impact. Forams contained in the samples dated the event at the late Eocene, 35 million years ago.
Then, in 1986, cores from boreholes from either side of the Chesapeake Bay revealed the presence of breccias - what had come up in Cedarstroms well tailings could be seen intact, and they were amazing - chunks from the size of bricks to automobiles and of all ages from Cretaceous (oldest sediments in SE VA) to Eocene were randomly interspersed in a muddy slurry the consistency of jello!
The following is the summary from "Chesapeake Bolide, Modern Consequences to An Ancient Cataclysm" - which can be found here on the USGS website:
http://woodshole.er.usgs.gov/epubs/bolide/index.html (http://woodshole.er.usgs.gov/epubs/bolide/index.html)
During the late Eocene, the formerly quiescent geological regime of the Virginia Coastal Plain was dramatically transformed when a bolide struck in the vicinity of the Delmarva Peninsula, and produced the following principal consequences:
The bolide carved a roughly circular crater twice the size of the state of Rhode Island (~6400 km2), and nearly as deep as the Grand Canyon (1.3 km deep).
The excavation truncated all existing ground water aquifers in the impact area by gouging ~4300 km3 of rock from the upper lithosphere, including Proterozoic and Paleozoic crystalline basement rocks and Middle Jurassic to upper Eocene sedimentary rocks.
A structural and topographic low formed over the crater.
The impact crater may have predetermined the present-day location of Chesapeake Bay.
A porous breccia lens, 600-1200 m thick, replaced local aquifers, resulting in ground water ~1.5 times saltier than normal sea water.
Long-term differential compaction and subsidence of the breccia lens spawned extensive fault systems in the area, which are potential hazards for local population centers in the Chesapeake Bay area.
A final note, when Dave Powars and Scott Bruce had proved the existance of the crater, they looked up Cederstrom, the geologist who first suspected its presence. He was in a nursing facility nearing the end of his life, but they were able to talk with him and show him the evidence that proved he was right, and he was able to understand what they told him.
[MOD EDIT - PLEASE PHRASE YOUR POST TITLES AS QUESTIONS, IN LINE WITH FORUM POLICY, THANKS, CHRIS]
-
Hi GRIT & welcome to TNS. As a GRIT, I feel I should point out that I am a BRIT, that's a double for Boy raised in Town & British!!!
This made an interesting read, and whole lot of questions. Doubtless JimBob and Bass will be greatly interested.
As for me, I wouldn't know a rock if it hit me on the head. (https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.freesmileys.org%2Fsmileys%2Fsmiley-violent037.gif&hash=8fbcef9800557e1515cf8a631e600fec) (http://www.freesmileys.org/smileys.php) OUCH! What was that?
-
No Ma'am - it is GRITS - with or without cheese (I done been raised down south, too. And Bass - Well, watch out, he was there when Sherman marched to the sea! He's still a little miffed about it. Got so angry had ta' leave Georgia!) And pay no attention to turtle boy. He is a bona-fide side-show freak.
Although I have been aware of this crater for a while, I did not know that it was causing problems in soil stability and subsidence today. What seems to be the consensus of opinion on the reason for this? Also, why is the water more salty - difference in seawater salinity with the melting of the glacial covering in the last 10-20,000 years?
Oh, Waterman girl, whaat's up on thaa baaay?
Lastly, and a tardy welcome to the site, Shell Hash Ma'am!
-
And pay no attention to turtle boy. He is a bona-fide side-show freak.
Oi!!!! I resemble that remark!!!
-
Hi GRIT & welcome to TNS. As a GRIT, I feel I should point out that I am a BRIT, that's a double for Boy raised in Town & British!!!
This made an interesting read, and whole lot of questions. Doubtless JimBob and Bass will be greatly interested.
As for me, I wouldn't know a rock if it hit me on the head. (https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.freesmileys.org%2Fsmileys%2Fsmiley-violent037.gif&hash=8fbcef9800557e1515cf8a631e600fec) (http://www.freesmileys.org/smileys.php) OUCH! What was that?
Hi Don,
I'm half Brit myself. My mother was born in Leyland, Lancs in 1909 and remembered being bombed by zepplins in WWI. (Lest you think I'm positively ancient, she was 42 when I was born.) I still have a cousin who lives in Suffolk, near Woodbridge, in a village called Bawdsey, which is at the mouth of the Deben. I visited her in 2004 and really enjoyed the country and the people.
Funny story, mom's memory of "the old country" remained stuck in 1928, when she immigrated to America to be a nanny/nurse. I was forever hearing how prim and proper English children were compared to my teen friends. So, when the Beatles were to appear on the Ed Sullivan show, she insisted I watch to see some refined music rather than that "infernal rock 'n roll". Off course, I had her on with "Awwww maaaa - do I hafta?" The look on her face when those mop heads took the stage would have stopped an 8 day clock!
Regarding that rock - incoming at about 50 grand an hour, you wouldn't have had time to put your head between your knees and kiss your butt goodbye!
-
No Ma'am - it is GRITS - with or without cheese (I done been raised down south, too. And Bass - Well, watch out, he was there when Sherman marched to the sea! He's still a little miffed about it. Got so angry had ta' leave Georgia!) And pay no attention to turtle boy. He is a bona-fide side-show freak.
Although I have been aware of this crater for a while, I did not know that it was causing problems in soil stability and subsidence today. What seems to be the consensus of opinion on the reason for this? Also, why is the water more salty - difference in seawater salinity with the melting of the glacial covering in the last 10-20,000 years?
Oh, Waterman girl, whaat's up on thaa baaay?
Lastly, and a tardy welcome to the site, Shell Hash Ma'am!
Yo, there, Jim-Bob - thanks for the welcome.
The reason for the subsidence and salinity is that the impact took place in a shallow sea. All of the existing strata were blown out, the tsunami is thought to have reached the Blue Ridge, then it all washed back into the hole. The heat evaporated much of the water, but what was left combined with loose sediments to make a gooey (like that geology term?) matrix. Also, listric faults occurred at the edges and large chunks of the rim slipped forward and down, into the crater. In the ensuing 35 mya, several more layers of strata were laid down (one of the larger being the Pliocene "Yorktown Formation" which happens to have a "Coquina facies" thus Coquina from Yorktown is an inside joke.) Anyway, the weight of the later strata "squished" (another sophisticated gem) the contents within the crater, causing the subsidence.
What's up on the bay? Boating's a little slow these days. I am bookkeeper for a marina that has exchanged the majority of its sales from new boat purchases to engine repair bills. Ethanol gas and fiberglass or aluminum tanks is not a good combo. Guess that's a topic for the Tech forum.
-
A topic and an idea from the designers that is DUMB as rocks! I should know - I live with a rfw of them. They don't seem to hear and talk back, either.
-
Fascinating stuff!
Do you know if there are any charcoal beds in the Eocene along the east coast? I believe that time was warm, humid rainforest- so just curious if there is any evidence of incineration of the forest?
-
A topic and an idea from the designers that is DUMB as rocks! I should know - I live with a rfw of them. They don't seem to hear and talk back, either.
Having been witness to the unresearched side effects caused by the introduction of ethanol gas, I hesitate to contemplate what will happen if the health care reform bill passes. Can you say "chaos theory"? They's a bunch of flutterbys flappin' their wings right now about 150 miles north of me.
-
For one thing, there may have been at least one other contemporaneous impact, in Siberia. I have read of charcoal beds in some areas, but since the impact has been associated with a large tsunami, anything regional would have been extinquished, post haste.
-
There - that's telling him!
(And very sound reasoning as well)
-
Local tsunamis, but not regional along the whole east coast. I would suspect massive fires in the rainforests along most of the east coast. There's some interesting recent research on the shape of impact craters and ejecta paths- will try to send references.
-
Local tsunamis, but not regional along the whole east coast. I would suspect massive fires in the rainforests along most of the east coast. There's some interesting recent research on the shape of impact craters and ejecta paths- will try to send references.
Here is a link to a book on the Geology and Geophysics of the Chesapeake Crater:
http://books.google.com/books?id=U5joncTT4voC&pg=PA372&lpg=PA372&dq=sandia+labs+chesapeake+crater&source=bl&ots=Etmn1cp0d5&sig=uEZURzy0hahuy_Kj_-6cxQRpHPI&hl=en&ei=pMedSrSpKYj-M4eXzYUC&sa=X&oi=book
If you go to page 372, section 12.2, you can read about a model of the impact generated at Sandia Labs - which was based on a submarine crater rather than a terrestrial one. Fig. 12.5 on page 375 shows the results of the model. It appears that the entire fireball was contained within the ejecta curtain of the crater. I suppose that doesn't preclude some fires started some distance away by debris that rained back down.
Unfortunately, I don't own this book, and haven't read the entire contents that is posted on the web, I will have to see if our local library has it. Anyway - I have read in other documents that the tsunami is thought to have reached the Blue Ridge - that is quite a wall of water as they are better than 150 miles inland, and that wall of water would have encompassed all 360 degrees. I have traveled the East Coast up I95 as far as Rhode Island, and know that the land up to CT does not have significant elevation - I would think the tsunami would have innundated anything at least that far.
I haven't heard Dave Powars mention anything about charcoal beds thought to be the result of this crater (though there is a "black mat" at the top of the Lower Dryas that may indicate an impact on the ice sheet at that time).
I don't claim to know all there is to know about this crater, and there may be some research that has been published of which I am not aware. If anyone has anything to contribute, please do so.
-
Not having yet read the link you posted- do you know if any paleo-channels or water gaps exist from the tsunami draining back into the ocean? Just curious
-
Not having yet read the link you posted- do you know if any paleo-channels or water gaps exist from the tsunami draining back into the ocean? Just curious
Page 70, Fig. 2.14, shows the studied area - the authors have noted a "washback channel?" in the area of the lower James River.
The effects on local waterways was one of the things that piqued my interest when I decided to research the crater. I grew up boating on the Chesapeake, and we participated in Navigation Contests - I was navigator and studied the channels and tidal currents in an effort to get a leg up on the competition. At the time, I thought some things seemed perplexing, but I couldn't put my finger on it.
Here's a large pdf file of the Chesapeake Bay Watershed:
http://chesapeake.usgs.gov/pubs/cbimage.pdf
You can see that both the James (most southern river) and the York (next one to the north) change direction close to their mouths. If you look at the land mass to the south of the James, you will note that the shoreline forms an arc between the mouth of the James and the Atlantic. If you complete that arc, you are approximating the crater rim, so you can see that the James and the York switched direction to flow to the topographic low. (This was noted in the first link I gave - "Ancient Cataclysm")
I know that there are faults in the main river channels, (there's a graphic of them in "Ancient Cataclysm" I also know that the James and the York both have depths of over 100' at the locations roughly corresponding the the direction change. This is speculation on my part -
If the faults were formed at the time of impact and extended to the surface (the Potomac formation of Cretaceous time is very thick in the location, due to sediment washed down from the Blue Ridge, which were much younger and steeper) the force of the tsunami backflow would have eroded them into topographic low features which would dictate the future water course.
Non-scientifically speaking, just using innate logic, there doesn't seem to be enough natural watershed to generate the volume and velocity of water needed to gouge out three major rivers so close to one another.